CN101157042A - Method and apparatus for catalytic oxidation and reduction of gases and vapors with crystalline compounds of heavy metals and rare earth elements - Google Patents

Abstract

The invention describes the preparation of catalysts for oxidation and selective reduction having properties similar to the catalytic properties of noble metal catalysts, prepared by forming crystalline surfaces on catalyst supports from synthetically crystalline, multi-step prepared, rare earth elements and elements of the metal components cobalt and/or lanthanum.

Description

Method and apparatus for catalytic oxidation and reduction of gases and vapors with crystalline compounds of heavy metals and rare earth elements

The present invention relates to a process for the preparation of such catalysts and their use in catalytic reaction systems as replacements for platinum catalysts and redox systems such as _DeNOx -synthesis.

DE 19800420A1 or US-A-4 707 341 disclose such catalysts as honeycomb or bulk (Schuett) layer catalysts. Here, an aqueous solution consisting of a first component with lanthanum, cerium and cobalt compounds, a second component of platinum, a third component of rhodium, and a fourth component of a wash-coat layer formed of aluminum oxide, titanium dioxide and oxalic acid is applied. on the catalyst.

The catalyst has the disadvantage that the active material has a low surface area, which can only be supplemented to a sufficient catalytic effect by the additional use of platinum and rhodium. Therefore, the compound formed from lanthanum, cerium and cobalt acts only as an improved surface on platinum and can increase the service life.

It has now surprisingly been found that when the active substance formed from rare earth elements and cobalt is crystallized in multiple steps, the platinum fraction is almost completely replaced, the service life is increased and the catalytic action is fundamentally improved. Here, although the substances cobalt and manganese and the rare earth elements lanthanum, cerium and yttrium are also used, crystals with a diameter of 1 to 0.1 μm and a length of 100 to 100,000 μm are reshaped in a specific multi-step process.

According to the method of the invention, this takes place in the following steps:

The salts of the rare earth elements lanthanum, cerium and/or yttrium and the metals cobalt and/or manganese are added in stoichiometric proportions to deionized water until a 20 to 60% solution is formed. Here, the stoichiometric ratio is the amount corresponding to the atomic weight of the material of the reaction participants added to the composite LaCoO ₃ , where La can be replaced by cerium and yttrium, and cobalt can be replaced by manganese. As salts, nitrates, carbonates and acetates are used.

The subsequent combustion process requires temperatures between 500 and 761°C. Set the highest temperature, since this temperature is the start of another recrystallization process and must be avoided. The coarse-grained mixture formed during heating was again dissolved in 20 to 60% oxalic acid until the sediment became less. This deposit was removed from solution by repeated decantation. The pure solution free of deposits is reheated to a combustion temperature of 500 to 600° C. and thus processed into a fine powder in crystalline form.

Now stir another aqueous solution of oxalic acid containing 20 to 40% oxalic acid. Molecularly fine aluminum oxide (Condea) 10% and fine Bayer titanium or equivalent _TiO2 are now introduced into this solution with stirring. Finally, the crystal powder prepared above was added to the solution. The mixture thus formed is vigorously stirred into a thin liquid suspension, possibly with further addition of water.

The suspension is used for the actual catalyst preparation by immersing the support with a honeycomb or bulk layer structure in the suspension. Here, the support must be completely immersed, since otherwise the binding effect would lead to an inhomogeneous distribution of the catalyst on the surface.

The wet catalyst thus obtained is processed in a "calciner" at 450 to 550° C. for at least 12 hours to produce the finished catalyst. In order to "kickstart" the catalytic reaction better, the catalyst is immersed in a solution of platinum nitrate or palladium nitrate after the combustion process, thus forming a catalyst with a platinum or palladium concentration of 0.1 to 0.5 g/l. The final catalyst thus formed also needs to be calcined once more at 450 to 550° C. for more than 6 hours.

The catalyst body formed has a uniform crystal structure or crystal layer on the surface, which is visible in a microscope or a scanning electron microscope. No point in concentration between rare earth elements and cobalt or manganese detectable in a scanning electron microscope (REM) is allowed to fall below said stoichiometric ratio. The rare earth elements must always be present at least stoichiometrically or superstoichiometrically. Likewise, the distribution of platinum or palladium on the crystal must be at least 30% and cannot be bound only to Condea or titanium dioxide. The crystal surface can be analyzed using a REM with laser analysis.

The invention is explained in more detail in specific examples. This example shows the formation of a crystalline catalyst of the present invention, ie, a catalyst coated with a crystalline layer having platinum-like properties, from the starting materials of the rare earth elements, lanthanum and cerium, and the heavy metal reactant partner cobalt. The purpose of this example is to prepare the complex La _0.9 Ce _0.1 CoO ₃ in such a way that no free cobalt is available to decompose the crystals and that the complex is completely neutral and non-toxic.

For this purpose, the ingredients lanthanum acetate, La(CH ₃ COO) ₃ ×nH ₂ O (the content of La ₂ O ₃ is 40%), cerium acetate, Ce(CH ₃ COO) ₃ ×nH ₂ O (the cerium content of CeO ₂ 45%) and cobalt acetate, Co(CH ₃ COO) ₂ ×4H ₂ O (cobalt content 24%) were mixed in the following manner:

430 g lanthanum acetate + 45 g cerium acetate + 250 g cobalt acetate were dissolved and stirred into a bath containing 3 liters of deionized water. The solution was brought to a temperature close to 100°C with stirring and then heated to 600°C without stirring. Here, these substances were reacted into powders containing the complex La _0.9 Ce _0.1 CoO ₃ up to 90% and 10% La _0.9 Ce _0.1 mixture.

The complex is stable due to excess La _0.9 Ce _0.1 and prevents excess cobalt from being active on heavy metal loading as well as thermal decomposition of the complex. Therefore, the excess of rare earth element complexes is an important feature of the formed catalyst crystals. 300 g of black catalyst powder were formed.

The catalyst powder was re-dissolved in a solution of 300 g oxalic acid and 3 liters of water under heating and heated again to 500°C to form a similar black powder, however, it had a very fine crystal structure and was very uniform under the scanning electron microscope element distribution. If other rare earth elements and manganese were added, the process was repeated a third time until complete homogeneity was achieved.

The black catalyst powder thus formed, 300g of needle-like crystal material, also called perovskite, is now stirred into 4 liters of deionized water containing 105g of oxalic acid, 100g of Condea and 50g of Bayer titanium, a fine-grained titanium oxide. The ceramic body to be wetted (treated as a catalyst by immersing it in the solution) is dried beforehand.

Honeycomb bodies, rolled metal bodies with general tubes and porous ceramic tube presses are suitable as catalyst bodies. As ceramic materials for the green body, materials magnesium-aluminum-silicate, cordierite, SiO ₂ -body, titanium-tungsten-oxide-honeycomb and aluminum oxide were prepared, wherein the material magnesium-aluminum- Silicates, cordierite, SiO ₂ -bulks, titanium-tungsten-oxide-honeycombs are particularly insensitive to expansion and are therefore particularly suitable.

The catalyst support is immersed in the catalytic solution in such a way that the porous body is coated uniformly, which means that it must be immersed completely and quickly so that the liquid can be capillary-infused into the interior of the ceramic with as little capillary action as possible. If the body is dipped too slowly or on one side, the capillary forces of the ceramic body will pick up the liquid and filter the catalytic species, coating unevenly.

After dip coating, the excess liquid is separated off. This is done by placing the body on a base covered with a screen, which absorbs excess liquid. After the liquid has completely settled in the lower part of the honeycomb body, it is removed from the liquid in the lower part by shaking or simply blowing off, so that the cells of the honeycomb body are left empty.

After coating, the catalytic coating is activated by calcining the body at a temperature of 500° C. for a period ranging from 2 to more than 20 hours, which means that the oxalic acid is completely burned off from the honeycomb. The burning time depends on the size of the honeycomb body. The larger the honeycomb, the longer the burn time.

A honeycomb body thus provided with crystal layers is not yet complete for catalytic action. It has been determined according to the invention that small amounts of noble metal which are subsequently applied in a separate coating process play a special role. The reason is that noble metals are mainly segregated on the crystals of La _0.9 Ce _0.1 CoO ₃ and La _0.9 Ce _0.1 . This is achieved by impregnation with a noble metal solution, such as platinum nitrate, at a concentration of 1 gram of platinum dissolved in 1 liter of deionized water, followed by burning the ceramic body also at 500°C.

This results in a very stable positioning of the noble metal compared to conventional noble metal coatings and a very high service life compared to coatings containing Condea and Bayer titanium. Furthermore, noble metals render La _0.9 Ce _0.1 CoO ₃ and La _0.9 Ce _0.1 coatings fully catalytically active faster than "ignition alloys", which means that the catalytic activity of platinum or other noble metals "ignites" the lanthanum-cerium-brilliant Catalytic activity of cobalt ores. This interaction also occurs with regard to catalyst poisons, which poison the two catalytic systems La _0.9 Ce _0.1 CoO ₃ and noble metals to varying degrees. Each less poisoned system activates the other.

As a result of the coating according to the invention, the following advantages arise, which are also based on the economy of the system. Comparable noble metal coatings that are equally active require 20 times higher amounts of noble metal than coatings of the invention, while only having a service life of about 5%, and the influence of catalyst poisons on the coatings of the invention is reduced.

Furthermore, the coatings according to the invention produce completely new and surprising effects in addition to the catalytic oxidation of hydrocarbons. The honeycomb body enables the selective removal of nitrogen oxides from the exhaust gas, which means that nitrogen oxides are reduced even in oxygen-containing exhaust gases and the remaining oxygen does not react with the lanthanum-cerium-cobaltite surface. However, the process only proceeds until the highest oxidation stage of the catalyst is reached. The catalyst must then be regenerated, which can be done with CO and _H2 according to the current state of the art.

Claims (9)

1. Process for the preparation of catalysts for the oxidation of gaseous and vaporous hydrocarbons (VOC) and the selective catalytic reduction of _DeNOx , characterized in that compounds of rare earth elements and heavy metals cobalt and manganese are used on a support in a multi-step crystallization process A layer of crystals is formed, which acts as a catalytically active species. 2. The method according to claim 1, characterized in that the reaction participants rare earth elements and heavy metals, cobalt and/or manganese, are formed in excess stoichiometric ratios so that an excess of rare earth elements is produced in the crystal layer. 3. The method according to claim 1 or 2, characterized in that the rare earth elements lanthanum, cerium and yttrium are used together or individually. 4. The method according to claim 1 or 2, characterized in that the metals cobalt and/or manganese are used as heavy metals. 5. Process according to claim 1 or 2, characterized in that the starting material is a salt dissolved in water and oxalic acid. 6. Process according to claim 1 or 2, characterized in that complexes of rare earth elements and heavy metals are formed by heating the mixture to a temperature of 500 to 761° C., and the process is repeated at least by dissolving in oxalic acid water once. 7. The method according to claim 1, 2 or 6, characterized in that the catalytically active substance is stirred into an impregnation solution in an oxalic acid mixture with the addition of molecular aluminum oxide, Condea and titanium oxide, Bayer titanium. 8. The method according to claim 1 or 2, characterized in that the final coated catalyst is activated by heating to 500°C. 9. A method according to claim 1, 2, 6 or 8, characterized in that the finished catalyst is once again immersed in a noble metal solution and then heated to 500°C so that the concentration of noble metal on the green body is 0.05 to 0.5 g/l.